Water holding tanks, such as aquaria and other vessels that form animal habitats, often recycle and reuse the water (e.g., saltwater or freshwater) within the tank. In some cases, several treatment techniques may be used to clean and treat the recycled water. For example, unwanted particulates may be removed and a chemical composition of the water may be adjusted. In some cases, animals that live in the tank, as well as the food in which they consume, are creators of harmful substances in the water, e.g., through uneaten feed decomposition and animal waste. Nitrate, for example, may be a byproduct of such uneaten animal feed and animal waste that, when occurring in undesirable concentrations within the water, can affect animal health.